If his backside is on the wire, then so too are those of nearly 20 other Cabinet ministers and, in particular, his boss.
Key has tried to defuse the controversy with thinly veiled insinuations Labour had something similar in train after it upset the investor in the first place by placing a ban on live-sheep exports following the 2003 Cormo Express disaster and ruining his import business.
After suggesting journalists put in Official Information Act requests for the Labour material, the Key Government delayed the release of the documents after officials suggested the publicity might jeopardise international relations.
Yet, the whole furore has already done that.
The Saudis would have thought they had achieved a nifty compensation deal for their investor via the secret deal. This involved a $4 million payment to effectively settle the dispute, another $1.5 million to fly sheep in and establish a breeding farm in Saudi Arabia, plus $6 million of services from New Zealand Trade and Enterprise to foster a so-called agri-business hub.
In February 2013, McCully told the Cabinet the deal would help secure the Gulf Co-operation Council free trade deal.
After six successful negotiating rounds, this had sat on the table since October 2009 awaiting signature and final ratification.
Key's positioning on this would be bolstered if the completed deal is unveiled soon. There have been expectations this might occur in either of the two trips he has made to the Middle East. He did not help relations by cutting short his first trip to come back to New Zealand after several soldiers were killed when their helicopter crashed into a hill.
That Key aborted his trip to come home when these men lost their lives as a result of an accident - not through being in the line of fire in a combat zone - was not understood in the Middle East. But in his latest whistle-stop trip, the deal still wasn't signed off.
Former Foreign Minister Phil Goff has been less damning of McCully than some other Opposition politicians. Goff has been around long enough in international circles to know there are a lot more grievous transgressions than a mere $11.5 million compensation deal.
As he told the Herald: "I'm keeping a partially open mind on it; it just looks like a cover for passing across $10 million or $11 million to the unhappy private investor."
It was legitimate for a foreign minister to unblock any obstacle, "but if you are going to do that, you have to be very careful indeed".
And there is the rub. The problem is that in putting forward his solution to compensate the aggrieved Saudi, McCully failed to nail it conclusively to the finalisation of the free trade deal. The usual avenue for this would be through an agricultural co-operation agreement that would sit as an appendix to the FTA.
In other words, all issues would be solved in tandem. That they weren't shows a surprising degree of ineptitude, which might have been avoided if the Cabinet had referred it to the usual control departments and Crown Law.
Debate over whether what was proffered by McCully was a facilitation payment or a bribe will continue. This will be heightened when the Organised Crime and Anti-corruption Legislation Bill, which continues to allow facilitation payments related to overseas transactions, is further debated in Parliament.
Transparency International New Zealand argues that facilitation payments are small bribes, and these are illegal in many jurisdictions, including Britain under its UK Bribery Act. It wants MPs to legislate to make bribes, no matter how small, illegal whether paid in New Zealand or overseas.
Its local head, Suzanne Snively, has called on MPs to safeguard New Zealand's reputation as one of the world's most corruption-free states by re-examining the issue as the bill progresses to its next stage.
"The big mistake is to view these 'facilitation payments' as being of little consequence," she says. "There is no such thing as a harmless bribe."
Snively's comments came before the Saudi saga erupted.
The Government will move to try to cover off this problem in coming weeks, but with Parliament again set to debate the legislation, the Cabinet's collective backside will be over the wire for some time to come.
